WASHINGTON  Greenhouse gases -- the heat-trapping chemicals linked to global warming -- continued to increase steadily in 2005, the U.S. National Oceanic and Atmospheric Administration reported Monday.

Carbon dioxide, emitted by coal-burning power plants and cars, increased last year, according to the federal climate agency's Annual Greenhouse Gas Index, or AGGI. So did nitrous oxide, a byproduct of farming and industry.

But methane emissions leveled off and chlorofluorocarbons, artificially-made chemicals used in refrigerators and air conditioners, declined, the agency said in a statement.

"Overall, the AGGI shows a continuing, steady rise in the amount of heat-trapping gases in the atmosphere," the agency said.

The AGGI showed a 1.25 percent rise in overall greenhouse gases in 2005. The index stood at 1.215 in 2005 compared to a 1990 base of 1.00, reflecting a steady rise in greenhouse gases over the past 15 years.

The index is based on the levels of greenhouse gases in the atmosphere in 1990, a year chosen because the global Kyoto Protocol that aims to limit emissions of these gases also picked it as a baseline.

The constant or dropping rates of methane and chlorofluorocarbons, also known as CFCs, have slightly slowed the overall growth rate.

By contrast, global carbon dioxide increased from an average of 376.8 parts per million in 2004 to 378.9 parts per million last year. The pre-industrial era level of carbon dioxide, a major influence on global warming, was about 278 parts per million.

A global network has measured levels of these chemicals, and chemicals that have replaced CFCs, since 1979. The first AGGI was released last year.

The increase of 1.25 percent in 2005 was relatively small compared to previous years, which have seen rises since 1979.

The largest annual increase was between 1987 and 1988, when levels jumped 2.8 percent; the smallest was .81 percent, from 1992 to 1993.

Greenhouse gases fluctuate naturally, but human and industrial factors play a role, and the rise in greenhouse gases has contributed to global warming.

Over the past 50 years, the average temperature on Earth has risen at the fastest rate in recorded history, with the 10 hottest years on record occurring since 1990.

The Bush administration was initially skeptical but now accepts the reality of global climate change, which has been associated with stronger hurricanes, severe droughts, intense heat waves and the melting of polar ice.

Last week, 10 states plus New York City and Washington, D.C. sued the U.S. Environmental Protection Agency, claiming new standards do not do enough to limit carbon dioxide emissions from power plants, despite what they say is clear evidence that this contributes to global warming.